Alternating current signal receiver



Oct.' 23, 1945. A, A. LUNDSTROM 1 2,387,671

ALTERNATING CURRENT SGNL RECEIVER Filed Oct. .'20, 1942 2 Sheets-Sheet 1 SENDER SENDER FIG..

LINE FINDER mi osrRoM BV it i.

Patented Oct. 23, 1945 CE: L

' l.s-L'ri:iuwvrlivo-cnninusvr SIGNAL RECEIVER Alexis A. Lundstrom, East Orange, N. J., assgnor i to Bell Telephone Laboratories, Incorporated, New York, N. Y1, aicorporation of New York A `Aii'iplirnttion October 20, 1942, Serial No. 462,706

(01; 1ro-E17) Y 4 Claims.

This invention relates to signal receivers and more particularly to an: alternating current signal receiver of the type disclosed in Patent No. 21,33212, issued on October 26, 15H3` to Messrs. G. Hecht, AJA. Lundstrom andE. Re Taylor. A broad object i the invention is to include in a receiver of 4the type mentioned and when utilized for-receivingalternati-ng current signal impulses from automatic telephone stations,4V means responsive to lthe first half cycle ofA succeeding incoming pulses ,to discriminate between noise-im duced. pulses pulses generated at the station and', also, to identify each of two party stations bridged across the same pair ofV line conductors. g lfhe receiver to which the present invention iS adapted as an improvement thereon comprises a plurality ot signal channels each responsive to a particular frequency n the signal frequency band and a' signal present"V .channel which is responsive to, all frequencies within the. band as wellasthose outside of the band. The latter channel, when activated, conditions `for operation" a reeist'erwhich is settable in response to the operation of two or the various signal channels when the same respond to pulses made up of two signal frequencies according to a code, theregister ignoring any pulse of one frequency asfhaving been produced by noise rather than 'The transmitting keys.` are, not, ordinarily, ex-

, nosed to noise and, therefore-,7. are protectedonly by the manipulation o f the signal producing instrument, andl closing the circuit `of a suitable atarrn for a received pulse of more than two fre- `quenci'es., the' alarm', in the latter case, being intended' to call attention to the fact that the signal producing' instrument probably has been incorrectly manipulated as, for instance, by pressing two keys at one time, since the production of more than two frequencies is more likely to arise from this cause rather than from any other.

` The receiver, as disclosed in the above-mentioned patent, is part of a complex circuit organization called the;` sender used inl telephone switching systems for registering wanted designationsand for selectively positioning switching apparatus in response thereto. The sender, in turn, is part ofan automatic telephone system wherein a calling line is extended to an operat'ors position from which it is further extended to a wanted line in other omces through the utilization thereat of a free sender vwhich is set by the codedplural frequency pulses designating the wanted member and, in response to the setting, selectively positions a. selector train to reach that line. The device for generating these pulses,A which is located at theY operators position, comprises a plurality of digit keys, the individual against such noise. as might occur,` for instance, at the contact of the switch` train ,over which the connection is established or at the contacts of lthe keys themselves. 1

In seeking to applysuch a receiver to a, sender for use in a telephone system in which the alternating current pulses of a wanted designation are generated by a device at a subscribers station rather than at one operators i position,` great care` must be exercised to guard the receiver against responding to noise generated currents Yinasmuch as: the transmitter ot the telephone set, being connected to thel line at the same time `as the receiver, and being exposed to the immediate surroundings,l will pick up whateverv sounds are present, which sounds, in affecting its diaphragm, will produce noise currents withinthe `frequencies to which the receiver is adaptedlrespend. Safeguards against such false operations are` usually providedlfor either through a manually operated key which disconnects the transmitter from the line during `pulsing `operations or through a rectiiying device in series with the transmitter which offers a very high loss to the noise generated currents Where the central oflice must be arranged for duplex operation, that is, for using common equipment to serve new lines equipped with, devices for producing alternating current pulses vand old lines equipped with a'dial for producing direct current pulses, the problem of discriminating against noise currents becomes greater. While the lines equipped for keying alternating current pulses are protected by rectifying cle-- vices against the generation and transmission of noise cmrents, the older dial lines, having no such or equivalent devices, have no such protection. Since the sender must serve bothtypes of lines indiscriminately, the receiver part of the sender may be exposed to false operation when the sender becomes connected to a dial equipped line since the sender; at the time of such connection, has no way of determining whether the line to which it is connected is a dial equipped line or a key equipped line. It is possible, of course, to provide each dial line with a rectifying network for the protection of the receiver, but the multiplying factor for such a network would make the cost prohibitive in comparison with the cost of providing adequate safeguards in the receiver itself, since there are less receivers than lines.

One of the objects of the present invention, therefore,V is an alternating current receiver for use in a sender adapted for the selective positioning of a train of switches, which shall be capable of discriminating between a calling dial line and a calling key line through the recognition of the'- polarity of the rst half cycle of the pulses produced by the calling key line.

It is another object of this invention to produce a receiver which shall be capable of discriminating between each of two party stations equipped with keying devices, through the recognition of the polarity of the first half cycle of the rst pulse produced by the station originating the call.

These and other objects of the invention may be more clearly ascertained from the following description, appended claims and attached drawings in which:

Fig. 1 shows two telephone lines, line A equipped with the usual dial and line B with party stations C and D connected thereto each having alternating current generating devices E. Both lines are shown as capable of connection in the known manner through crossbar switching facilities to a sender selector through which an idle sender becomes connected to the calling line. Inasmuch as the extension of a calling line to an available sender through crossbar switches (or other types of switches) is old in the art and forms no part of this invention, the same is only conventionally indicated;

Fig. 2 shows parts of the receiver disclosed in the G. Hecht et al. patent as modified to include the elements which constitute the present invention.

The alternating current generating device E with which stations C and D are provided is of the type disclosed in Patent 2,147,710 issued to R. F. Mallina on February 21, 1939 but is provided with an additional or sixth reed. The six reeds are disposed between the poles of a permanent magnet, each of the reeds having a 'natural period of vvibration equal to the periodicity oi one of the six signal frequencies which, for convenience, may be designated a-f. Surrounding the reeds is a coil which is connected to the line conductors, and a key-set having ten digit keys is so mounted with respect to the reeds that the depression of any key will pluck two of the reeds and set them into vibration within the eld Aof the magnet thereby generating in the coil currents each having a frequency determined by the natural period of one of the reeds. The depression ofY different keys will, of course, result in the plucking of different reeds according to a prearranged code so that each digit will be indicated by a pulse of two distinct frequencies. ity of the first half cycle of the current produced by the depression of any key will be determined, of course, by the manner in which the coil of the devices E are connected to the line. Where they are connected in the same sense, the polarity of the rst cycle of the current produced by each device will be the same, that is, either both positive or both negative. Where they are connected in the opposite sense, the polarity of the first half cyc1e of each current will be opposite. Since., in

The polarthe present invention, it is contempleted that the receiver shall recognize the identity of each of two party stations equipped to produce plural frequency pulses by the polarity of the first half cycle of the first pulse generated, the coil of device E for station C is connected across the line opposite to that of device E for station D.

Referring, now, more particularly to Fig. 2, there is shown the volume limiter VL which, on the input side is coupled to the preceding high pass filter HPF connected to the manually ad- `justable pad AP and, on the output side, to the receiving channels indicated by the vacuum tubes Ta--Tf, each channel being reserved for response to the frequency designated by the subscript designation of the tube of that channel. Tapped across the resistance NI is the primary winding of transformer SPT to the secondary winding of which is connected to the grid of the vacuum tube S of the signal present channel. All of these elements of the circuitare the same as those disclosed in the above-mentioned patent to G. Hecht et al. and have been herein similarly designated as, also, have been similarly designated theelements shown connected to the signal channel tubes Ta-Tf except that the connections to the contacts of the check polarized relay CK2 and the signal pr'esent relay SPR havev .been slightly changed for operation with the present invention.

In paralle1 with the primary winding of transformer SPT is the primary winding of transformer NPT. This transformer has two secondary windings connected in series, and to the mid-point thereof is connected the negative pole of the battery supply BS, as is also connected the common terminal of condensers CNI and CN2 and the potentiometer resistors Rl, R2 and R3. The upper terminal of the series connected transformer windings is connected over an undesignated resistor to the grid of gas-filled tubev TA while the lower terminal is similarly connected, also through an undesignated resistor, to the grid of gas-filled `tube TB.

relay Bl and resistor R6, all'of said resistors and relays being joined together and to thefcommon terminal of potentiometer resistors R2 and R3 whence is obtained the positive potential derived from battery BS to operate the tubesTA and TB and the relays AI and. .BI as hereinafter` described. i l rf v The armatures of relays Al and Bl obtain ground through the armature and'lower contact of relay CK2, and this ground is normally connected to the two windings of the relay G via the lower contacts of relays AI and BI. Inas` much as the common terminal of the two windings of relay G is grounded via th armature and lower contact of the signal present relay SPR, relay G is normally short-circuited, the battery BP for supplying current to the windings of said relay G having a path to ground through resistors R1 and R8 and the aforesaid lower contacts ofrelays Al and BI.

Relays A2 and B2 are-local relays operating over the front contacts lof relays Al and BI, respectively, and Iwhen either of the latter relays operates, it connects ground to an individual conductor which extends to a relay in the register. Since, as will beshown shortly, re-

asoman lay A2' signifies'` that party CV calling and relay B2 signies that party D is calling,y it is obvious that when either:l relay operates: to operate its locally controlled. relayin the register, said register is informed of the party identifica.- tion of the calling line.

Having described the component. elements. of the circuit. of my invention its mode of oper ation will now be described.

Assume, first, that the cnice isarranged for duplex operation and that, therefore, linestypif fled by dial line A and party line; B1 are both to be served by the samecentral office equipment, Now let it be assumed that station C of` line B initiates a call by removing the receiver from the hook and that, in consequence, the line is extended over the line finder switches and sender selector switches to an idle sender', designated at the selector by conductors l and 2 and that portion of its equipment shown in Fig. 2. By means oi well-known operations,l the Somier theny proceeds to transmit a. tone over the calling loop to indicate to the calling station'- that the wanted number maybe keyed (or dialed).

It will be remembered` that, in the case of two party stations, the magnetizing. coils of the signal generating instruments EV are oppositely connected to the line for each station so` that the operation of av key each mstrument will cause the production ot an alternating current= ot opposite polarity to that produced by the other. Thatis,l if. We assume `that the coi-l` of` device E at station D is wired so that the operation, of a key of the device will'. generate a eurrenthaving a positive first lobe, then the coil of device E at station Cy is wired so that the operation ci a key of the latter devicewill generate a current having a negative first. lobe. We may now `assume for simplicity that the energizing coils of both devices E are so Wired. that the depression. of a key of device E at station C `produces a current which consists ci two superimposed frequencies (depending upon the code' used) within. the sig nal frequency band each of which. has anrst negative lobe. Since the circuit iu` completed over conductors I- and 2 through. the primary winding ot transformer IT, the signal voltage is induced the secondary winding of said transformer and applied to the grids of the tubes of.

the volume limiter VL whence, according to the c teachings ofV the above-mentioned Ge Hecht et al. patent, the signal energy is ampliiied (or reduced its intensity is4 above the ceiling of the limiter); and applied to the channel lters BPFBPF whence it is directed through the filters tuned to the frequencies in. the signal pulse to cause the operation of the corresponding signal channels..

The portion of the signal voltage which is available across resistor NI is applied to the primary winding of transtormer SPT or the signal present channel and. to the primarywinding of transformer NPT. The former results in the operation of the tube S and. relay` SPR while the latter results in the` operation. oi either tube TAv or tube TB depending upon the. polarity oi the rst half cycle of the incoming pulse.` It the polarity oi the rsthalf cycle is negative `(las assumed), the voltage induced inthe secondary of .transformer NPT will be oi opposite polarity .so that the voltage thereof will be positive at the upper terminal of the secondary windings'- and negative at the lower terminal., This positive voltage is applied to the grid of tube TA, overcomes the: negative bias thereat from battery BS over theupper winding of the transformen. and renders the voltage on the` grid, suiciently pooli-4 tiveto fire the tube.. In consequence a flow ot current is then established over thel path extending froin the positive polef of battery; BSf resistor R3, Winding of relay Al and paralleled resistor-R5; the cathode-anode spacer path with--` in: tube TA, resistor R41, resistor Rt to the. nega tive pole oi battery BS.. The; firing of tube' TA. and the. current. flowing. in consequence. thereof'` through the` above-traced circuit immediately-` causes a highY negative bias to be applied tothe. grids of both tubes TA and 'IB over the rela,-` tively high resistance` R4 which is located in the common anode and grid circuit path. The establishment of. this` high negati-ve bias: locksl out tube TB since anyv positivepotential which may be thereafter applied to the gridxthereot, will notbe suicient to overcome the bias so that. for` any particular signal pulse,` one and only one or the two: tubes TA and TB can bef The current owing through the winding ot relay Al will cause its operation, theparallel path` throughv resistor RTS being provided.A merely to insure the fast establishment oi the abovementioned `lock-out negative bias;

In the.` same manner, when sta-tion'D initiates a call and aA key oi the device E thereat is. de pressed, then. since` the magnetizing coil of this device is connected to the lineoppositely trom that of the device at station. C the rsthalt cycle of the generated current will be positive,A con.- sequel-icef of which a positive potential is induced at the: lower terminal of the secondary winding of transformer NPT and tube TB is caused. to fire in the same mannerasmalread-y described for tube TA. l

Up to this point any signal of a frequency above the cut-ofi. frequency of band-pass filter HPF and of sufiicient magnitude will cause the operation: ot either relay Al or Bl depending upon. the initial polarity ot the rst. received pulse. From hereon however, the opera-tion of the circuit depends largely on the reactions ot the. receiver as; determined by the operzitionv of relays SPR,` CK2 and 0K3, relay SPR operate ing. upon any frequency passing through the lmiter VL, relay CK2 operating when any received pulse `cor-itains-two signal frequencies, and. relay 0K3 operating when any received, pulse: contains three or more signal. frequencies.' With these re.- lay operations in mind four circuit. reactions are possible. ,A

The first reaction. is that the. received pulse may be of such a short duration thatnone ofthe above relays will operate. The pulse, however, can never be short enough` to prevent the uri-ng of tube TA or TB and the operation of their respec-f tively associated relays At and Bl since the pulse need be only a. fractional part of a half cycle to alter the potential on the grid or either tube. It will be observed that with relay SPR normal and relays Al and Bl also norniaLrelay G is maintainedshort-circuited. However, upon the operation of either relay Al or Bl,v shortcircuiting ground removed from resistor R1 or R8, causes relay G to operate on the winding in series with the resistor Ry--from which the; short circuit has been removed. For example, if relay AI is operated, ground is disconnected from resistor Rl and a circuit is completed for relay G extending from battery BR, resistor Rl, right windingof relay G, armature and lower contact of relay SPR to ground. `0n the other hand, if relay Bt is operated, ground is disconnected from resistor R8 and relay G will then operate through its left winding over an easily traceable circuit. The operation of relayy G short-circuits the potential applied between the anode and cathode of both tubes TA and TB, causing whichever one is conducting to become extinguished, in turn causing the release of its locally operated relay AI or BI. Thus, if the pulse is too short to operate relay SPR, the circuit is returned'to' normal without registering any party designation. In other words, a signal which does not operate the signal present channel of the receiver but does iire either tube TA or TB of the 'polarity-detector is ignored.

The Vsecond condition to be noted is that which results in the operation of relay SPR but not of relays CK2 or CK3. This results from a signal impulse which, though of sufficient duration, has but one frequency. For this condition', the operation of relay SPR merely delays, for the duration of the signal pulse, the operation of relayG, at which time the operation of the latter relay causes the conducting tube TA or TB to be restored as above described. This delay is clesirable because it keeps the operated tube in a conducting state for the duration of the particular signal pulse pending the possibility of an operation of two or more signal channels Ta-Tf. However, should two or more of these channels fail to operate, relay G will operate upon the release of relay SPR and restore the ignited tube to normal.

'Ihe third condition to be noted is that which results in the operation of relay SPR and relay CK2.VK This is the normal condition responsive to a pulse of sufcient duration and composed of two frequencies since each digit or character transmitted from the calling station is assumed to be represented by a combination of two out of the six signal frequencies af-f to which the signal channels I'a-Tf are respectively adapted to respond. If two of the channels Ta-Tf are operated, then, according to the teaching of the abovementioned G. Hecht et al., patent, relay CK2 is operated. Remembering that relay AI is operated (or relay Bl if the call is initiated by station D), then, since the operation of relay CK2 disconnects ground from its lower contact, a circuit is completed for relay A2 which extends from ground through the right winding of relay A2, upper contact and armature of relay AI, resistor R9, to battery BP. Relay A2 operates, eX- tends ground over its No. 2 contacts and conductor A3 to a re1ay"(not shown) in the register which operates to record the fact that party C is making Vthe call, and completes a locking circuit for itself from battery BL, through resistor RIU, No. l contacts, conductor 4 to off-normal ground in the register, said off-normal ground being applied to said conductor by the register when the latter has registered the character or digit designation indicated by the two operated signal channels Ta-Tf.

When the iirst signal pulse has terminated, relaysA SPR and CK2 release and since relay AI is still operated, relay G is now operated in the manner described to cause the extinguishing of tube'TA. Relay A2, however, remains locked for the duration of the signaling period, that is, during the transmission of the remaining signal pulses from station C. These signal pulses will, of course, cause-tube TA to be rered and extinguished with each pulse, cause relay Al alternating to operate and release and thereby cause battery'BP alternately to be applied to and disconnected from the right winding lofrelay A2. However, since this relay is locked up under the control of the register, no further action takes place since the register has noted the party registration as party C by virtue of the ground applied to conductor A3.

'Ihe condition above noted for operating two signal channels and the signal present channel may be equally s'atised by speech or noise currents if the latter contain any two of the signal frequencies. The circuit, however, discriminates between noise and actual signal operation by the fact that the signals, originating at station C, comprise a series of alternating current pulses all of whose first half cycles have identical polarity. Since the probability of the noise or speech pulses of just two frequencies havingidentical polarity is extremely small while that of having some of the pulses of one polarity and other pulses of opposite polarity is extremely high, the noise pulses will cause the operation of both tubes TA and TB (after each tube is extinguished by relay G subsequent to its operation) and the consequent operation of relays A2 and B2 in the manner described. The operated condition of relays A2 and B2 causes ground tobe applied to conductor Y which extends to a circuit (not shown) in the register to inform the latter that the receiver has operated on speech or noise currents, Since no speech or noise currents are possible from stations transmitting alternating current impulses because their transmitters are appropriately guarded by rectiers or other circuit or mechanical means, the elements which operated both branches of the polarity detector could only have come from a telephone line using a dial such as station A since the transmitter of such a line is not guarded against noise or speech pick-up. Under these circumstances, ground on conductor Y informs the register that the calling line is a dial rather than a key line and proceeds in a manner not herein shown to remove the alternating current receiver portion of the sender from conductors l-2 and to replace it by the direct current pulsing relay for response to the forthcoming dial pulses.

The fourth condition to be noted is that which results from the operation of relays SPR; CK2 and CK3, which occurs when three or more signal channels are operated as a result of the inadvertent displacement of two keys of the keyset associated with the device E or if transmitter currents due to speech or noise are on the line. The operation of relay CK3 indicates the abnormal condition of three or more simultaneous channel operations. As described in the abovementioned patent, the operation of relay CK3 immediately causes recorder tone to be furnished to calling subscriber. Since these operations and the means for carrying them out form no part of this invention, the same are neither shown nor described herein.

Invthe duplex area, if dial pulses appear rst at the sender, it is recognized immediately by virtue of the operation of the pulsing relay as a dial 4phone and the alternating current receiverl equipment is then immediately removed and information received via the pulsing relay is used for switching operations.

What is claimed is:

1. A polarity detector for the first half cycle of an alternating current wave comprising two gasfilled electronic devices, each of said devices including a grid, a cathode and an anode, a source of potential connected in parallel between the cathode and anode of each device, a power transfer device serially connected between the grids of said two devices, means responsive to an alternating current wave applied to said power transfer device for rendering the grid of one of said devices positive with respect to the potential of the cathode thereof whereby said device is rendered conducting, means responsive `to the conductivity of said device for applying a high negative bias to the grid of said other device whereby the resultant potential on said grid due to the application of a positive half cycle to the power transfer device remains relatively negative so as to render said other device unresponsive to said positive half cycle, and a relay in the anode circuit of each` device responsive to the ow of current therethrough for short-circuiting the anode potential to said device whereby the latter is restored to its original responsive condition.

2. A circuit for response to plural frequency alternating current pulses comprising in combination a channel adapted for response to each frequency in the pulse, and a channel adapted for response to the rst half cycle of each pulse, a register settable in response to the operation of said frequency-responding channels, and means responsive to the operation of said last-mentioned channel in response to two pulses each of whose first half cycles are of opposite polarity for transmitting a signal to said register.

3. In a telephone system, the combination with a. plurality of lines, some of which are equipped with a dial device to transmit direct current pulses when calling and others are equipped with alternating current generators for producing plural frequency pulses indicative of wanted line designations when calling, of a sender comprising an alternating current receiver having a channel adapted for response to each frequency in a pulse and a channel adapted for response to the first half cycle of each pulse, and means responsive to the operation of said last-mentioned channel in response to two pulses each of whose first half cycle is of opposite polarity for indicating to the sender that the calling line is equipped with a dial and not with an alternating current generator.

4. In a telephone system, the combination oi a two-party station line, each of the stations on said line being equipped with an alternating current generator for producing plural frequency 

